The vast majority of railroad wheels in use today are formed entirely from 0.6 to 0.7 percent plain carbon steel cast in permanent graphite molds. Although formed from a single material, the railroad wheels have different portions that are subjected to different stresses during use and ideally should have different characteristics. For example, the rim of a railroad wheel is in continuous contact with a railroad track and should have good wear resistance. In general, both the rim and body should resist wear and fatigue, but the required wear resistance properties of the rim can compromise the desired level of fatigue resistance of the body portion. Similarly, the desired level of fatigue resistance of the body can reduce the rim wear resistance.
In an attempt to achieve the desired characteristics for the rim and body portions formed from a single material, these portions are processed differently. Conventional railroad wheel fabrication includes processing the rim portion of the wheel through a quenching process. That is, the rim is austenitized at a temperature in a range from about 1700 degrees to 1800 degrees Fahrenheit by rapid cooling with a liquid, such as water spray. The rim quenching process provides a fine perlitic microstructure for the steel and a desirable residual compressive state of stress in the rim about a circumference of the wheel. The fine perlitic structure is harder, and thus generally is more resistant to wear than coarser microstructures and the residual compressive stress on the wheel rim resists cracking due to fatigue and other stresses.
While quenching the rim portion of the wheel provides some contrast in the properties of the body and rim portions of the wheel, there is a limit to the different characteristics that a single material can achieve. Furthermore, there is an inherent tradeoff that is made to balance the desired properties of the different wheel portions.
In addition to providing differing characteristics for the rim and body of the wheel, a fabrication process for making the railroad wheel should be readily adaptable to provide a wheel rim having high traction or low traction depending upon the intended application. For example, a wheel for a locomotive should have a relatively high coefficient of friction with respect to a railroad track for enhancing the load pulling characteristics of the locomotive. Conversely, a freight car should have wheels with a relatively low coefficient of friction for more efficient transport of the freight car.
It would be therefore be desirable to provide a railroad wheel having enhanced wear properties, resistance to fatigue, shelling, and hot spotting. It is further desirable to provide a railroad wheel that can be readily fabricated for relatively high and low traction applications.